A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Imaging of the pattern onto the substrate is performed by a projection system, which may comprise a plurality of lenses or mirrors.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):CD=k1λ/NAPS  (1)
where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source in a lithographic apparatus. EUV radiation sources are configured to output a radiation wavelength of around 13.5 nm. Thus, EUV radiation sources may constitute a significant step toward achieving printing of small features.
It is conventional to monitor the optical performance of parts of a lithographic apparatus in order to ensure that the accuracy with which patterns are projected onto substrates remains high. For example, an imaging detector may be located in a substrate table of the lithographic apparatus, and may be used to monitor aberrations present in a projection system of the lithographic apparatus. In a conventional (non-EUV) lithographic apparatus, the imaging detector may comprise a CCD array that has been provided with a layer of a scintillation material, e.g., Gd2O2S:Tb (known as P43). The scintillation material converts radiation at, for example, about 248 nm or 193 nm into visible radiation at, for example, about 550 nm (or some other suitable wavelength). The visible radiation is then detected by the CCD array.
While it is possible to detect EUV radiation in the same way (using a CCD array provided with a scintillation material layer), the efficiency and/or accuracy with which the EUV is detected may be poor.